1. Field of the Invention
The present invention relates to a variable capacitance circuit capable of greatly changing capacitance by application of DC (direct current) bias voltages, while minimizing capacitance change, noises and nonlinear distortion due to radio frequency signals.
The present invention also relates to a variable capacitance thin film capacitor including the foregoing variable capacitance circuit formed on a supporting substrate.
The present invention further relates to radio frequency devices using the forgoing variable capacitance thin film capacitor, including voltage controlled radio frequency resonator, voltage controlled radio frequency filter, voltage controlled matching circuit chip, voltage controlled antenna duplexer and the like.
2. Description of the Related Art
There is a conventionally known thin film capacitor whose upper and lower electrode layers and dielectric layer are formed of thin films. Usually, this is fabricated by stacking lamellar layers including a lower electrode layer, a dielectric layer and an upper electrode layer in this order on an electrically insulative supporting substrate. In such a thin film capacitor, the lower electrode layer and upper electrode layer are deposited by sputtering, vacuum deposition or the like, and the dielectric layer is deposited by sputtering, the sol-gel process or the like. In the manufacture of such a thin film capacitor, a photolithography process as described below is usually used.
First, a conductor layer serving as the lower electrode layer is formed all over the insulative supporting substrate, and then only desired portions are masked with a resist. Thereafter, unnecessary portions are removed by wet or dry etching, thereby forming a lower electrode layer with a predetermined pattern. Subsequently, a dielectric layer serving as the thin film dielectric layer is deposited all over the supporting substrate, and then, in the same way as the lower electrode, unnecessary portions are removed to form a thin film dielectric layer with a predetermined pattern. Lastly, a conductor layer serving as the upper electrode layer is deposited all over the surface, and unnecessary portions are removed to form an upper electrode layer with a predetermined pattern. In addition, a protective layer and solder terminal portions are formed on top of the stacked layers. Through these steps, the thin film capacitor becomes ready to be surface-mounted on a circuit board.
There is also a known variable capacitance thin film capacitor, which employs (BaxSr1-x)TiyO3-z as the material for the thin film dielectric layer, in which a predetermined bias potential is applied between the upper and lower electrode layers so as to vary the dielectric constant of the dielectric layer, thereby varying the capacitance of the thin film capacitor. The structure thereof is similar to the foregoing one. A variable capacitance thin film capacitor is disclosed, for example, in the patent document 1 (Japanese Patent Laid-Open Publication No. 1999-260667).
In variable capacitance thin film capacitors, the dielectric constant is varied by application of DC bias, and consequently, the capacitance is varied. Change in capacitance also occurs in a radio frequency region, so that they can be used as variable capacitance thin film capacitors at radio frequencies.
By utilizing such capacitance change of the variable capacitance thin film capacitors at radio frequencies, electronic devices whose frequency characteristics can be varied by application of DC bias can be produced. For example, in a voltage controlled thin film resonator combining the forgoing variable capacitance thin film capacitor and a thin film inductor, the resonant frequency can be varied by application of DC bias. In a voltage controlled thin film bandpass filter combining the variable capacitance thin film capacitor or a voltage controlled thin film resonator with a thin film inductor and a thin film capacitor, the bandpass range can be varied by application of DC bias. An example related to voltage controlled electronic devices for microwaves is disclosed in the patent document 2 (Published Japanese translation of a PCT application No. 1996-509103).
When such a variable capacitance thin film capacitor as described above is used in a radio frequency electronic device, DC bias voltage for varying capacitance and voltage of radio frequency signal (radio frequency voltage) are simultaneously applied to the variable capacitance thin film capacitor. If the radio frequency voltage is high, the capacitance of the variable capacitance thin film capacitor is caused to change also by the radio frequency voltage. When such a variable capacitance thin film capacitor is used in a radio frequency electronic device, capacitance change in the capacitor due to radio frequency voltages will produce waveform distortion and noises caused by intermodulation distortion.
In order to minimize waveform distortion and noises caused by intermodulation distortion, capacitance change caused by radio frequency voltage needs to be minimized by reducing the intensity of the radio frequency electric field. For this purpose, increasing the thickness of the dielectric layer is effective. However, increasing the thickness of the dielectric layer causes the intensity of direct current electric field to decrease, which leads to the problem that the capacitance change ratio is also reduced.
Since, electric current easily flows through the capacitor at radio frequencies, a resistance loss in the capacitor causes generation of heat leading to breakdown of itself. To deal with the power handling capability problem as above, increasing the thickness of the dielectric layer so as to decrease the calorific value per unit volume is also effective. However, as described above, since increasing the thickness of the dielectric layer causes the intensity of direct current electric field to decrease, this also poses the problem of reduction in capacitance change ratio by application of DC bias.
Meanwhile, in the manufacture of thin film capacitors, generally, layers having other functions such as a protective layer and a solder diffusion barrier layer are successively stacked in addition to the lower electrode layer, thin film dielectric layer and the upper electrode layer. However, as the number of layers increases, in addition to problems such as misalignment in the photolithography process and damage to the lower layer during etching, stress is enhanced by the increase of the number of the layers, resulting in cracking in the films, which leads to undesirable characteristics and degraded reliability.
An object of the present invention is to provide a variable capacitance circuit and variable capacitance thin film capacitor in which capacitance change caused by radio frequency signal is small and capacitance change caused by DC bias is large.
Another object of the present invention is to provide a variable capacitance thin film capacitor in which capacitance change caused by radio frequency signal is small and capacitance change caused by DC bias is large, wherein the size of the device is maintained even when a new element such as bias lines is added and the number of successively stacked thin film layers is lessened, so that miniaturization and higher integration of the device are effectively achieved, and undesirable characteristics and degradation in reliability are prevented.
A still another object of the present invention is to provide radio frequency devices using the variable capacitance thin film capacitor such as voltage controlled radio frequency thin film resonator, voltage controlled radio frequency thin film filter, voltage controlled matching circuit chip, and voltage controlled thin film antenna duplexer which cause little intermodulation distortion and have high power handling capability.